1. Field of the Invention
The present invention relates to a method and apparatus for separating out metal copper according to an electroplating of copper using, for example, a solution of copper sulfate in order to fill copper in fine interconnection grooves formed in a surface of a substrate such as a semiconductor wafer to produce copper interconnections on the surface of the substrate.
2. Description of the Related Art
According to a conventional electroplating for plating a substrate with copper using a solution of copper sulfate, a substrate is dipped in sulfuric acid or the like so as to be activated by the acid in a pre-treatment process outside of a plating tank. Instead of such pre-treatment process, a substrate having a seed copper layer as an electrically conductive layer may be brought into contact with a solution of copper sulfate in the plating tank. A thin surface Cu layer etching without electrical current loading (de-energization) can be made for a certain period of time (activating time) in a pre-treatment process. Then, an electric current is supplied to separate out metal copper on the substrate after the pre-treatment process.
The former pre-treatment process is disadvantageous in that a tank different from the plating tank is necessary to carry out the pre-treatment process. Hence, the required facility is large and the running cost is increased.
On the other hand, the latter pre-treatment process is disadvantageous in that the plating solution and the seed copper layer on the substrate are not brought into contact with each other under constant conditions. Hence, additives such as a copper separation accelerator and a copper separation inhibitor contained in the plating solution tend to suffer initial adsorption irregularities to the surface of the seed copper layer and activation irregularities thereof Further, the substrate is susceptible to the specific adsorption of a component caused by a black film on a soluble anode positioned in confronting relation to the substrate. As a consequence, the metal copper is abnormally separated out locally on the surface of the substrate, causing the substrate to have a stained appearance. When the metal copper is nonuniformly and abnormally separated out locally, the crystal orientation of the copper and the thickness of the copper layer become irregular, making it difficult for the substrate to be polished to a flat finish by a chemical mechanical polishing (CMP) process after the plating process.
According to conventional solutions to the above problems, the activating time is increased, or the substrate is rotated or the plating solution is stirred by a device known as a squeegee, whereby adsorption irregularities and activation irregularities are eliminated. However, the activating process carried out for a long period of time tends to dissolve away the seed copper layer in its entirety because the seed copper layer provided as a fine interconnection pattern or a very thin electrode layer on the bottom of holes having a high aspect ratio is etched more than other portions, possibly making it impossible to embed metal copper according to electroplating. The other solutions referred to above are disadvantageous in that they make the entire system complex or large in size.
Further, the conventional copper plating process is problematic in that the thickness of the deposited copper film differs from location to location because of the presence of the interconnection pattern. According to this problem, specifically, the thickness of the deposited copper film is much larger in an area where fine interconnections are closely spaced than in an area which is free of fine interconnections. The hump, which is the difference between the thickness of the deposited copper film in the area where fine interconnections are closely spaced and the thickness of the deposited copper film in the area free of fine interconnections, may reach 1 xcexcm. The hump presents difficulty in polishing the deposited copper film to a flat finish in the chemical mechanical polishing (CMP) process subsequent to the plating process. Any undesirable remaining copper film in the area where fine interconnections are closely spaced tends to cause a short circuit between the interconnections. Thus, the yield of substrates is likely to be lowered.
It is therefore an object of the present invention to provide a method and apparatus for plating a substrate with copper which can prevent metal copper from being separated out locally on the surface of the substrate, allow a plated copper film to be easily planarized in a chemical mechanical polishing (CMP) process after the plating process, and finish the substrate to a mirror-like glossy surface with a relatively simple facility and a process.
In order to achieve the above object, according to one aspect of the present invention, there is provided a method for plating a substrate with copper, comprising: bringing, at least once, a substrate into contact with a processing solution containing at least one of organic substance and sulfur compound which are contained in a plating solution; and bringing the substrate into contact with the plating solution to plate the substrate.
The substrate is brought into contact with the processing solution before the substrate is plated and/or while the substrate is being plated. The phrase xe2x80x9cwhile the substrate is being platedxe2x80x9d means while the plated film is being deposited in a stage before the thickness of the plated film reaches a final target thickness for the plated film.
In the above plating method, before the substrate is plated, the substrate is brought into contact with the processing solution which contains at least one of an organic substance and a sulfur compound which are contained in the plating solution. Alternatively, after the plating solution is removed from the substrate by interrupting plating of the substrate, the substrate is brought into contact with the processing solution.
The substrate may be brought into contact with the processing solution by directly dipping the substrate into the processing solution in a tank, spraying the processing solution over the substrate while the substrate is being rotated in a horizontal plane at a high speed as with a spin dryer, or supplying the processing solution by a pump into a dedicated dipping chamber in which the substrate is set at a predetermined position. When the substrate is thus brought into the processing solution, a thin film of the organic substance and/or the sulfur compound is coated on the processed surface of the substrate. Extra processing solution is preferably removed from the substrate, and then the substrate is plated with copper according to a conventional process. In this manner, metal copper is prevented from being separated out locally on the processed surface of the substrate, and the substrate is plated to provide a mirror-like glossy surface. Further, the size of humps in an area of closely spaced interconnections on the processed surface of the substrate can be suppressed.
Thereafter, it is preferable to remove the processing solution from the substrate and/or to dry the substrate to minimize any amount of processing solution carried into the plating solution to maintain a better quality of the plating solution. However, since the amount of processing solution which is coated is usually much smaller than the amount of plating solution, removing the processing solution from the substrate and/or drying of the substrate are not necessarily required. The processing solution may be removed from the substrate by simply lowering the level of processing solution, lifting the substrate out of the processing solution, rotating the substrate to spin off the processing solution from the substrate as with a spin dryer, rotating the substrate and applying a nitrogen gas blow to the substrate, or passing the substrate through a forced air flow such as an air blower. Further, the two processes including removing of the processing solution and drying of the substrate may be performed successively by one apparatus. For example, the processing solution may be sprayed over the substrate while the substrate is being rotated by a spin cleaner/dryer or the like. This method allows the substrate to contact the processing solution and also allows the processing solution to be removed from the substrate.
The processing solution may be continuously removed from the substrate until the substrate is dried to a certain extent for thereby further minimizing the amount of processing solution carried into the plating solution. In this case, it is preferable to dry the substrate to a partly dried state with a certain moisture content, rather than fully drying the substrate.
The mechanism of the present invention, though it is not fully elucidated, is as follows. The organic substance used in the present invention, which is contained in the plating solution, is known to be effective in offering surface activity and suppressing the separating-out of copper for uniform electrodepositability. The sulfur compound, which is contained in the plating solution, is known to be effective in increasing the separating-out of copper to make the crystal of the separated-out film fine for thereby increasing the glossy level of the plated film. By coating a thin layer of the organic substance and/or the sulfur compound on the processed surface of the substrate in advance and/or while the substrate is being plated, the separating-out of copper over the substrate in its entirety is uniformly accelerated or suppressed, and any abnormal separating-out of copper is prevented. This effect remains the same after the substrate is dried to a certain extent.
Moreover, the organic substance or the sulfur compound in the plating solution is effective to increase wettability between the plating solution and the processed surface of the substrate. Even after the substrate is dried to a certain extent, this effect of the organic substance or the sulfur compound remains the same because the organic substance or the sulfur compound is eluted into the plating solution. Therefore, wetting between the plating solution and the processed surface of the substrate is improved and made uniform over its entire surface, allowing the entire surface of the substrate to be plated uniformly and efficiently. These advantages lead to an improvement of the embeddability of plated copper into high aspect ratio holes and grooves on the substrate.
The organic substance preferably comprises polyethers in an organic polymer for use in copper plating processes. Experiences in tests conducted by the inventors indicate that the organic substance has a concentration ranging from 10 mg/l to 10 g/l in the processing solution and a molecular weight ranging from 100 to 100,000. The organic substance may be a copolymer or a block polymer such as polyethylene glycol, polypropylene glycol, polyvinyl alcohol, ethoxy-naphthol, propoxy-naphthol, ethoxy-phenol, propoxy-phenol, polyoxyethylene polyoxypropylene block polymer, ethoxy-nonylphenol, carboxymethylcellulose, or polyethylene proplylene glycol. The processing solution containing the organic substance is particularly effective as a pre-treatment solution for use prior to the plating process.
The sulfur compound is represented by the following general formula:
Xxe2x80x94Lxe2x80x94(S)nxe2x80x94Lxe2x80x94X
where L represents a lower alkyl group, a lower alkoxy group, a hydroxyl group, or an alkyl group replaceable with a halogen atom and having a carbon number ranging from 1 to 6, x represents a hydrogen atom, an SO3M group, or a PO3M group where M indicates a hydrogen atom, an alkaline metal, or an amino group. The processing solution containing the sulfur compound is highly effective to accelerate the separating-out of copper in fine interconnections and suppress humps on the substrate. It is particularly effective that the sulfur compound has a concentration ranging from 0.1 xcexcmol/l to 70 xcexcmol/l in the processing solution.
The substrate and the processing solution may be held in contact with each other for a period of time sufficient to cause the processing solution to contact the entire surface of the substrate. If the period of time were too long, the current supplying layer (seed layer) would be chemically damaged. Usually, the period of time is selected in the range from 3 to 60 seconds. If the processing solution is strongly alkaline, then the hydrolysis of the organic substance and the sulfur compound tends to progress. If the acidity of the processing solution is too strong, then the copper of the seed layer is liable to be etched. For this reason, the pH of the processing solution is preferably in the range of 2 to 9. Depending on the type of plating apparatus used, a dried or partly dried substrate may be needed. In such a case, the above effect is unchanged even if the substrate is dried after the substrate and the processing solution are brought into contact with each other.
The method may further comprise etching the plated film at least once by interrupting plating of the substrate while the substrate is being plated, and the substrate is brought into contact with the plating solution again to plate the substrate after the plated film is etched. If the plated film is etched by interrupting plating of the substrate while the substrate is being plated, then the plated film may be etched by an electrolytic etching process in which the current is passed in a direction opposite to the direction in normal plating, or a chemical etching process in which sulfuric acid is employed. Further, after the etching process, the process for bringing, at least once, the substrate into contact with the processing solution may be performed, and then the substrate may be brought into contact with the plating solution again to plate the substrate.
The current supplied in the electrolytic etching process may be a direct current or a pulsed current (so-called PR pulse). The etched depth is proportional to the supplied amount of current (the product of the magnitude of the current and the time in which the current is passed). The current is supplied to the substrate at a current density ranging from 1 to 30 mA/cm2 for a period of time ranging from about 0.5 to 30 seconds. The concentration of sulfuric acid employed in the chemical etching process is preferably in the range of about 0.5 to 30%, and the substrate is held in the sulfuric acid for a period of time ranging from about 1 to 30 seconds. The sulfuric acid is a most popular additive added to the plating solution, and can easily be handled from the standpoint of the composition management of the plating solution. The thickness of the plated film that is etched away is 1 nm or more for achieving any appreciable effect of the etching process, and is preferably in the range of about 10 to 50 nm.
According to another aspect of the present invention, there is provided an apparatus for plating a substrate with copper, comprising: a device for bringing a substrate into contact with a processing solution containing at least one of an organic substance and a sulfur compound contained in a plating solution; and a device for bringing the substrate into contact with the plating solution to plate the substrate.
According to still another aspect of the present invention, there is also provided an apparatus for plating a substrate with copper, comprising: a device for bringing a substrate into contact with a processing solution containing at least one of an organic substance and a sulfur compound contained in a plating solution; a device for bringing the substrate into contact with the plating solution to plate the substrate; and a device for etching a plated film deposited on the substrate.
A processing tank for bringing the substrate into contact with the processing solution therein may be separate from a plating tank for plating the substrate, or one common tank may be used instead of these two tanks. If the two tanks are separately employed, then these tanks may be positioned closely to each other, with a delivery unit provided for quickly delivering substrates to the tanks. If one common tank is employed, then the tank may be combined with supply passages for individually supplying the processing solution and the plating solution, and drain passages for changing the solutions. The apparatus may further comprise a device for spraying the processing solution over the substrate while the substrate is being rotated, thereby to bring the processing solution into contact with the substrate. Therefore, the rotational speed of the substrate is increased to remove the processing solution from the substrate and/or to dry the substrate.
The apparatus may further include, in addition to the processing tank and the plating tank, a loading and/or unloading unit for loading and/or unloading substrates, a transferring device for transferring substrates, a cleaning unit for cleaning substrates, and a drying unit for drying substrates, so that substrates can be loaded and unloaded in a clean condition.